1. Field of the Invention
2. Description of the Related Art
This invention relates to a method for adjusting a continuous dialysate volume flow in a dialysis machine and a dialysis machine for carrying out the method. The dialysis machine in particular can be a peritoneal dialysis machine.
In the automatic peritoneal dialysis a dialysis machine controls and monitors the introduction of the fresh dialysate into the abdominal cavity and the discharge of the used dialysate. Such dialysis machine, also referred to as cycler, usually fills and drains the abdominal cavity several times during the night, i.e. while the patient sleeps.
A peritoneal dialysis machine according to the prior art is shown in FIG. 14 by way of example. As shown here, two pumps 50 usually are employed, which include diaphragm pumps as pump actuators 51. The same act on pump chambers with which dialysate is pumped from correspondingly present dialysate bags into the abdominal cavity of a patient and used dialysate is discharged from the abdominal cavity of the patient. To achieve a constant dialysate volume flow despite the discontinuously operating diaphragm pump, the hydraulic pressure Phyd in the hydraulic lines of the diaphragm pumps is determined. For the case that the diaphragm pumps are driven pneumatically, the corresponding pneumatic pressure in the lines is determined. To ensure pressure monitoring, the pressures Phyd measured by means of the pressure sensors 55 must be compensated by some influence values. These are on the one hand the respective diaphragm pressure Pdiaphragm, i.e. the back pressure which is caused by the deflection and internal tension of the diaphragm in response to the measured hydraulic pressure Phyd. With increasing deflection the diaphragm tension increases disproportionately, which is accompanied by a constructionally caused velocity reaction. This back pressure depends on the position L of the hydraulic pump 58, which usually is measured via a displacement transducer 56. In addition, the back pressure PFR, which is caused by the flow resistance in the system, i.e. in the pump 50 and in the pump chamber 53 designed as disposable, is taken into account as further compensation variable. This back pressure to be taken into account is dependent on the velocity v in the system.
Finally, the hydrostatic pressure Pstat must be taken into account, which results from the position of the patient.
The procedure usually is as follows:
Initially, the diaphragm compensation is measured. The hydraulic pump 58 travels along the entire working path and picks up the existing pressure value in equidistant intervals, which are verified by the displacement transducer 56, and plots the same in a curve. This curve provides for compensating the originally measured raw value of the hydraulic pressure Phyd by the back pressure of the diaphragm Pdiaphragm. The velocity compensation, i.e. the back pressure PFR which is caused by the volume flow resistance, is firmly stored in the method and need therefore not be measured. For the configuration, the same is determined in advance and stored in a corresponding memory.
The detection of the hydrostatic patient pressure Pstat is possible at the beginning of each outflow phase. When filling the dialysate or when discharging the dialysate, the system usually will specify a desired volume flow for the phases “Fill/Drain”. The objective is to produce a continuous flow. Achieving this objective is rendered more difficult by using two discontinuous pumps.
As shown in the velocity/path diagram of FIG. 15, an individual pump chamber is accelerated at the beginning of the pumping stroke and decelerated at the end of the pumping stroke. During the acceleration phase, i.e. the so-called ramp-up, and the deceleration phase, i.e. the so-called ramp-down, the pumped dialysate volume flow varies. To avoid this, as shown schematically in FIG. 15, the ramp-down of a first pump chamber is superimposed by the ramp-up of the second pump chamber such that a constant dialysate volume flow is obtained.
According to the prior art, the specified volume flow of the dialysate is adjusted at the pump described above by means of the displacement transducer 56. However, this leads to a change in pressure of the entire system, which must be evaluated. When a limit value PPatMax is exceeded, the movement of the pump is stopped. This limit value corresponds to the exceedance of a maximum permitted patient pressure limit. To obtain a continuous volume flow in operation of the two pumps, a ramp-up of the volume flow is performed in the chamber ends, as explained above (cf. FIG. 15). Due to this actuation, the following prerequisites and properties are obtained in the system:
First of all, the diaphragm tension Pdiaphragm at the beginning must be picked up once as curve over the entire range in dependence on the pump position.
This means that the determination of the patient pressure is characterized by this compensation, which leads to inaccuracies, as the length signal of the length sensor to be taken into account only is comparatively difficult to determine.
Since the velocity is adjusted continuously, the system has the property to increase the energy employed in the controller in response to closures of the patient feed line or in the case of a “drained” patient (towards the end of a cycle). This will inevitably cause the exceedance of the previously indicated limit pressure RPatMax.
Finally, the internal tension of the diaphragm is increased very much at the chamber ends and acts against the velocity control, whereby the superposition during start-up or shut-down of the pump systems is rendered more difficult.
What is disadvantageous in the prior art system for adjusting the continuous dialysate volume flow in particular are the high requirements to be satisfied by the control system or the measurement system. Since these requirements frequently are not satisfied to the desired extent, a non-uniform run of the hydraulic pumps can occur.